The present invention relates to a method for operating an internal combustion engine, in which the fuel is pumped from a first fuel pump to a second fuel pump and, from there, into a high-pressure region, from where the fuel passes into at least one combustion chamber of the internal combustion engine via at least one fuel injection device, and in which the pressure in the high-pressure region is lowered in certain operating conditions of the internal combustion engine by means of a release device.
A method of this type is made known in DE 195 39 883 A1. According to that publication, when the internal combustion engine is switched off, pressure compensation is established between the pressure side of the second fuel pump and a fuel tank and/or ambient pressure. This is carried out via a fuel line in which a valve is located, the valve being configured as a pressure-control valve when in the energized operating position, and as a flow restrictor when in the de-energized operating position. Systems are also known in which only a flow restrictor is provided.
These measures serve to effectively prevent fuel from passing into the combustion chambers of the internal combustion engine through the fuel injection devices after the internal combustion engine is switched off. This unburned fuel would result in increased emissions when the internal combustion engine is started.
The release device has other advantages as well: it prevents fuel from passing from the fuel injection devices into the internal combustion chamber of the internal combustion engine when the internal combustion engine operates in overrun, and it prevents an impermissible pressure increase in the high-pressure region after the internal combustion engine is switched off, which is caused by heat being conducted from the engine block and warming the fuel that is located in the high-pressure region. In addition, a pressure limiter, which limits the pressure in the high-pressure region, can also be configured with a simpler design due to the release device that is proposed. Furthermore, if maintenance must be performed, the pressure can be relieved on the pressure side in a simple manner so that parts can be safely removed, if necessary. In addition, when the pressure is reduced, pressure dynamic properties are improved.
During operation of the internal combustion engine, however, it was determined that difficulties during start-up of the internal combustion engine and an impermissibly high pressure increase in the high-pressure region cannot always be ruled out with absolute certainty.